Anti-skid system

ABSTRACT

This relates to an electronic control system for an anti-skid system having one braking pressure control valve actuated by the control system to regulate the braking pressure applied to the wheel brakes of both wheels of one axle of a vehicle. The control system includes a speed sensor associated with each wheel. A reacceleration detector, an incipient skid deceleration detector and a speed decrease detector is coupled to each sensor and a subtractor is coupled to both sensors. A first series circuit including a first AND gate and a first time delay device is coupled to one of the incipient skid deceleration detectors and one of the speed decrease detectors. A second series circuit including a second AND gate and a second time delay device is coupled to the other of the incipient skid deceleration detectors and the other of the speed decrease detectors. First logic circuitry is coupled to each of the time delay devices and the subtractor to produce a first actuation signal for the control valve to release the braking pressure in both wheel brakes when both of the incipient skid deceleration detectors and both of the speed decrease detectors having an output signal or when the subtractor has an output signal, one of the incipient skid deceleration detectors has an output signal and one of the speed decrease detectors has an output signal. Second logic circuitry is coupled to each of the time delay devices and each of the reacceleration detectors to produce a second actuation signal for the control valve to reapply braking pressure to both wheel brakes when both of the reacceleration detectors have an output signal and both of the time delay devices have an output signal, or when one of the time delay devices has had no output signal and the other of the time delay devices has had an output signal and the associated one of the reacceleration detectors has an output signal.

United States Patent 1 1 Fink et a1.

[1 1 1 3,756,663 [451 Sept. 4, 1973 ANTI-SKID SYSTEM lnventors: WernerFink, Frankfurt; Erwin Schlitz, Heusenstamm, both of [21] Appl. No.:206,009

[30] Foreign Application Priority Data Jan. 18,1971 Germany P 2102 131.8

[52] U.S. Cl. 303/21 BE, 303/20 [51] Int. Cl. B60t 8/08, B60t 8/12 [58]Field of Search 188/181; 303/20, 303/21; 324/160-162; 340/52 R, 262-263[56] References Cited UNITED STATES PATENTS 3,672,730 6/1972 Burckhartet a1 303/21 BE 3,260,555 7/1966 Packer 303/21 EB 3,606,492 9/1971 Hayes303/21 EB 3,653,727 4/1972 Kullberg et al.... 303/21 P 3,482,887 12/1969Sheppard 303/21 BE 3,556,610 1/1971 Leiber 303/21 P 3,620,576 11/1971Wehde et a1... 303/21 BE 3,640,589 2/1972 Taniguchi 303/21 BE 3,671,0836/1972 Matsumura 303/21 BE and MenottiJ. Lombardi,J r. et a1.

[57] ABSTRACT This relates to an electronic control system for anantiskid system having one braking pressure control valve actuated bythe control system to regulate the braking pressure applied to the wheelbrakes of both wheels of one axle of a vehicle. The control systemincludes a speed sensor associated with each wheel. A reaccelerationdetector, an incipient skid deceleration detector and a speed decreasedetector is coupled to each sensor and a subtractor is coupled to bothsensors. A first series circuit including a first AND gate and a firsttime delay device is coupled to one of the incipient skid decelerationdetectors and one of the speed decrease detectors. A second seriescircuit including a second AND gate and a second time delay device iscoupled to the other of the incipient skid deceleration detectors andthe other of the speed decrease detectors. First logic circuitry iscoupled to each of the time delay devices and the subtractor to producea first actuation signal for the control valve to release the brakingpressure in both wheel brakes when both of the incipient skiddeceleration detectors and both of the speed decrease detectors havingan output signal or when the subtractor has an output signal, one of theincipient skid deceleration detectors has an output signal and one ofthe speed decrease detectors has an output signal. Second logiccircuitry is coupled to each of the time delay devices and each of thereacceleration detectors to produce a second actuation signal for thecontrol valve to reapply braking pressure to both wheel brakes when bothof the reacceleration detectors have an output signal and both of thetime delay devices have' an output signal, or when one of the timedelaydevices has had no output signal and the other of the time delay deviceshas had an output signal and the associated one of the reaccelerationdetectors has an output signal.

13 Claims, 4 Drawing Figures .iaecd Decrease Detector W/Qeacm/emtmnDeteclor PATENTEBSEP 41915 SHEET 3 OF 4 ANTI-SKID SYSTEM BACKGROUND OFTHE INVENTION The invention relates to an anti-skid system for motorvehicles in which, in order to prevent one or more wheels from skidding,the braking pressure in the brake cylinders of the wheels of at leastone axle is influenced jointly by a single pressure control unit as afunction of the signals of speed sensors detecting the state of motionof the wheels.

Anti-skid control systems of the. above mentioned type are known indifferent embodiments.

U.S. Pat. No. 3,498,683 mainly suggests an arrangement of an inlet andan outlet valve in the pressure medium line between the master cylinderand a wheel brake cylinder, said inlet and outlet valve being actuatedby acceleration and deceleration signals of sensors provided at thewheels. When the rotative deceleration of a wheel reaches a certaincritical value, which would cause skidding of this wheel if the brakingwas continued with undiminished force, the inlet valve is closed and thepressure medium connection between master cylinder and wheel cylinder isinterrupted. At the same time the outlet valve is opened, therebyconnecting the wheel cylinder to a pressureless reservoir so that thepressure medium can be released in the part of the brake line which isconnected with the wheel cylinder.

Thus, the wheel reaches reacceleration after having passed adeceleration maximum and the critical rotative deceleration value isagain exceeded. At this point only the outlet valve is actuated andclosed again while the inlet valve remains closed so that during thefollowing period the pressure medium largely remains constant in thepart of the brake line connected with the wheel cylinder. By the inertiainherent to the system the wheel is reaccelerated and approachesthe'spee d of the vehicle after having passed an acceleration maximum.The constant pressure in the brake line is maintained until the wheelhas reached a speed favorable for a new braking. At this time the inletvalve is opened at the corresponding acceleration signal of the sensor.Then a new braking and a new control cycle is started.

It would be obvious that a voluminous construction would result byinserting an inlet and an outlet valve in each of the wheels to becontrolled as well as providing an adequate electronic control systemfor their actua-' tion. This would result in each wheel being controlledindividually, and pressure would be released in the wheel cylinder ofeach wheel only as far as is necessary to prevent this one wheel fromskidding. In practice this possibility would not be considered due tothe costs and space involved, especially since the wheels of one axlenormally reach the state of skidding approximately at the same time,therefore, both valve units must be actuated simultaneously.Consequently, a valve and an electronic control system will be providedin the common brake line of the wheels of one axle, that is, to saybefore the bifurcation. The control system will then be actuated by thesensor signal of the wheel reaching the critical deceleration valuefirst. As long as the friction value between road and wheel contactsurface of the wheels is nearly equal, or the difference is so smallthat it can be neglected, this is the only profitable manner ofproviding an anti-skid control system.

The case may occur, however, that during a braking there are, forexample, friction values of such difference at a left and a right wheel(contaminated road or partly covered with ice) that there aresubstantial disadvantages for the brake action if the control iseffected according to the principle described above. The wheel with thelower friction value reaches the critical deceleration levelconsiderably earlier than the other wheel or wheels. The control actionand, thus, the pressure release is initiated by the wheel having thelower friction value. The pressure release does not only apply to theskidding wheel, but also to the wheels which still roll orderly and canbe braked impeccably. The braking distance is considerably longer due tothe early pressure release in the brake line of these wheels which arenot skidding. If, however, the deceleration level is set too high, or ifthe response of the control system depends on the sensor signals of bothwheels acting on its input, there is the possibility of the decelerationlevel being exceeded and the danger of skidding not being overcomesafely. I

The same disadvantages also occur in anti-skid systems which control thebrake pressure differently, but for at least two wheels in common as afunction of deceleration and acceleration levels, or speed levels.

U. S. Pat. No. 3,401,987 describes a device in which a closing valve anda plunger unit operable together with the closing valve are arranged inthe pressure medium line of one axle, preferrably the rear axle, beforeits bifurcation to the two wheel cylinders. The plunger is connectedwith a spring-loaded diaphragm which can be actuated on both sides as afunction of the sensors detecting the state of motion, especially thestate of deceleration, of the two wheels of this axle so that theplunger is moved accordingly and the brake pressure introduced in thewheel cylinders by the driver canbe decreased and increased again.

Only one pressure modulator is provided for both wheels of one axleinthe system according to the U. S. Pat. No. 3,331,641. A valve,actuated in accordance with deceleration, controls the pressure suppliedby a hydraulic source of energy which acts on a piston in oppositedirection, said piston being operated by an attendant or driver.According to the position of the piston the hydraulic source of energyis connected with the wheel cylinders for the actuation of the brakes,or this connection is simply interrupted, thus, maintaining a constantpressure, or there is a connection to the wheel cylinders allowing apressure release in the whole brake circuit. The deceleration sensor aswell as the valve controlled in accordance with deceleration areprovided for each wheel. In a certain deceleration range, the brakepressure is continuously throttled as a function of the deceleration ofall four wheels due to the effect of force and counterforce at thepistons controlling the brake pressure. As soon as a wheel approachesthe critical deceleration value, the brake pressure in the whole brakecircuit is influenced by acting on the piston opposite to the pedalforce.

All of these known anti-skid control systems, which modulate the brakepressure in the wheel cylinders of the wheels of at least one axle, havethe above mentioned disadvantage for the case in which the difference ofthe friction values between road and wheel contact surface of theindividual wheels cannot be neglected during a braking.

SUMMARY OF THE INVENTION of a single braking pressure control valve forthe wheel brakes of at least one axle. This electronic control systemcan be employed with many different anti-skid braking pressure controldevices to overcome the above-mentioned disadvantage, but yet will notexcessively increase the costs for the anti-skid system or the spacerequired to house the same.

A feature of the present invention is the provision of an anti-skidsystem for a motor vehicle comprising a single brake pressure controlmeans to control the braking of both wheels of at least one axle of thevehicle; and an electronic control system to produce at least a firstactuation signal for the control means to release braking pressure atboth of the wheels to prevent skidding of at least one of the wheels;the control system including a pair of speed sensors, a different one ofthe sensors being coupled to each wheel of the one axle, and first meanscoupled to both of the sensors and the control means to produce thefirst actuation signal as a functlon of speed signals and incipient skiddeceleration signals generated in response to a speed signal from bothof the sensors.

Another feature of the present invention is the provision of theabove-mentioned control system further including second means couPled toboth of the sensors and the control means to produce a second actuationsignal for said control means as a function of reacceleration signalsgenerated in response to a speed signal from both of the sensors, thesecond actuation signal causing the control means to reapply brakingpressure to both of the wheels.

The control system produces the first actuation signal when both wheelsof at least one axle have reached a predetermined incipient skidrotative deceleration level b, and from this moment on a decrease of therotative speed Av at both wheels.

Furthermore, the control system produces the first actuation signal whenone of the wheels has reached a predetermined incipient skid rotativedeceleration level -b, and also a decrease Av of the rotative speedthereof and a rotative speed difference A has been detected between thetwo wheels of the one axle.

The control system produces the second actuation signal when both wheelsof at least one axle have reached their respective reaccelerationmaximum b or when the deceleration of one wheel of an axle previouslyreached the deceleration level -b and now has reached its reaccelerationmaximum b, but the second wheel has not reached the deceleration levelBRIEF DESCRIPTION OF THE DRAWING Above-mentioned and other features andobjects of this invention will become more apparent by reference to thefollowing description taken in conjunction with the accompanying drawingin which:

FIG. 1 illustrates the speed and deceleration curves of two wheels ofone axle of a vehicle during a control cycle with approximately equalfriction value at both wheels and also the switching action of thecontrol valve;

FIG. 2 illustrates the speed and deceleration curves of two wheels ofone axle of a vehicle with a difference of the friction value at eachwheel, which cannot be neglected, during a control cycle and also thecorresponding switching action of the controlling valve;

FIG. 3 illustrates the speed and deceleration curves of two wheels of avehicle with a great difference in the friction value at each wheelduring a control cycle and also the corresponding switching action ofthe controlling valve; and

FIG. 4 illustrates in schematic block diagram form one embodiment of theelectronic control system for an anti-skid system in accordance with theprinciples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Diagram A of FIG. 1 illustratesthe curve 1 of the vehicle speed v during a brake action versus time 2.solid line curve 2 illustrates the corresponding curve of the rotativespeed v,; of a right wheel of one axle of a vehicle and dotted curve 3illustrates the curve of the rotative speed v of a left wheel of the oneaxle of this vehicle versus time t. FIG. 1 illustrates the case whereduring a brake action the resulting skid control the wheels of the leftand right side of the vehicle have largely the same friction valueconditions, and, consequently, the rotative speeds of the left and rightwheels have no or only a slight difference.

Diagram B of FIG. 1 illustrates the acceleration (deceleration) curve bof the same wheels as graph A over the time t, said curves occurringduring the brake action. The solid line curve 4 applying to the rightwheel and the dotted line curve 5 applying to the left wheel. Curve 6 indiagram C of FIG. 1 represents the switching action of the control valvedisposed between master cylinder and wheel cylinder of a vehicle axle,said switching in the present case being effected as a func tion of thespeed and acceleration conditions at the wheels on-both sides of thevehicle. At I the control valve for the pressure release in the part ofthe brake line connected with the wheel cylinders of an axle is closedand at II this control valve is reopened for new pressure build-up inthe same part of the brake line. The switching action of the controlvalve is not efi'ected as in the systems according to the prior art,that is to say, only as a function of deceleration and accelerationlevels. The electronic control system of the present invention producesa first actuation signal for closing the control valve at I when therotative deceleration of both wheels of the controlled axle havingreached a certain deceleration level -b, and the rotative speed of bothwheel have been reduced by a fixed amount Av while the rotativedeceleration still increases by the amount Ab. The diagrams of FIG. 1illustrate these difference amounts marked Av and Ab for the left wheeland Av and Ab, for the right wheel. The difference of the rotativespeeds Av and Av, is equal as to its absolute value while thedifferences Ab;, and Ab of the rotative deceleration can have anegligible difference. After the separation of the wheel cylinders fromthe master cylinder by the closing of the control valve as describedabove the brake pressure is released either by pressure mediumextraction or by volume increase in the part of the brake line connectedwith the wheel cylinders so that the wheels reaccelerate after a certaindeceleration time due to inertia and their rotative speed againapproaches the vehicle speed. Both wheels having reached a certainacceleration level, preferably their reacceleration maximum b theelectronic system produces a second actuation signal, said secondactuation signal reopening the closing valve at II in curve 6 of FIG. I.This enables building up new brake pressure in the wheel cylinders andenables the start of a new control cycle.

By this kindof response of the control system to a combined signal,which takes into cosideration the state of motion of both vehicle wheelsof an axle, a too early initiation of the control at a lowerdeceleration level for the vehicle and an excessive increase of thebraking distance is prevented on the one hand. On the other hand theexceeding of the critical deceleration level and the skidding of thevehicle are also prevented as long as the friction value conditions atboth wheels only differ by a certain maximum.

FIG. 2 shows the corresponding curves in the diagram A, B and C for thecase where friction value differences occur at the right and the leftwheel of a vehicle axle during a braking which cannot be neglected.Curve 11 in diagram A of FIG. 2 illustrates the speed curve versus timeof the vehicle. Curves l2 and 13 represent the simultaneously occurringrotative speeds of the right and left wheels of the vehicle axle. In therepresented example it is assumed that the right vehicle wheel (solidline curve 12) finds a substantially smaller friction value than theleft vehicle wheel (dotted line curve 13). Accordingly the rotativespeed at the right wheel decreases considerably faster after applicationof the brakes and the right wheel tends to skid earlier than the leftwheel of the same vehicle axle. Diagram B of FIG. 2 illustrates theresulting strong deceleration at the right wheel (solid line curve 14)and the substantially smaller deceleration at the left wheel (dottedline curve 15).

If in this case with the different friction value conditions at theright and the left wheel of a vehicle axle the control was initiatedaccording to the same criteria as described above with respect to FIG.1, namely, beginning the decrease of the brake pressure only as soon asboth vehicle wheels have reached a certain decelerationlevel b, and apredetermined speed difference Av, then the right wheel would havelocked in the represented example long before the left wheel would havereached the deceleration level b,. The switching of the control valve,preferably in the form of a 3/2-way valve, shown in curve 16 of diagramC between master cylinder and the wheel cylinders as well as theaccompanying decrease and new increase of the brake pressure in the partof the brake circuit constantly connected with the wheel cylinders mustbe effected as a function of another signal combination.

A second electronic working cycle in the electronic control systemproduced a first actuation signal closing the control valve at[ in thecurve 16 as soon as one of the two wheels (in the represented examplethe right one) reaches the acceleration level b, and from this moment onshows a speed difference of Av, and as soon as there is a difference Abetween the rotative speeds of the two wheels. At this point the closingvalve is actuated (point I in the curve 6), i.e. closed, so that thewheel cylinders of this axle are separated from the master cylinder, andthe brake pressure in the part of the brake circuit connected with thewheel cylinders is released by pressure medium extraction or volumeincrease. The inertia inherent to the system causes a furtherdeceleration at both wheels for the time being. The left wheel, too,then reaches the deceleration level b,, a fact which does not, however,influence the control. Both wheels pass a deceleration maximum and thenare reaccelerated so that their rotative speeds approach the value ofthe present vehicle speed. The previously less decelerated wheel in ourexample the left one, dotted curve 13 and 15) actually reaches thisvalue and possibly rotates at vehicle speed for a limited time,completely without slip, until the control valve is reopened at point IIin the curve 16 by a new signal produced in the control unit. Thissignal is produced as soon as the left as well as the right wheel havereached their reacceleration maximum b and b in this case at twodifferent spaced times. After the opening of the control valve pressurecan be built up again from the master cylinder and a new brake controlcycle begins. Thus, on one hand there is prevented, with unfavorablefriction conditions for a vehicle wheel, that the brake pressure in thewhole brake circuit, or in the brake circuit of one axle is drecreasedunnecessarily early and the braking distance prolonged and, on the otherhand, there is also prevented the skidding of a wheel before the controlsystem responds.

FIG. 3 shows the speed and deceleration diagram as well as the switchingof the control valve for an extreme case of friction value differencesat the right and the left wheel of a vehicle axle.

In our example, the right wheel again has an extremely low frictionvalue between road and wheel contact surface, and the rotative speed ofthis wheel decreases rapidly after braking is started (solid line curve22 in diagram A). The left wheel, however, can continue to roll and itsrotative speeds drops only slightly in the same periodof time (dottedline curve 23 in diagram A) and remains near the vehicle speed (curve21). The deceleration curve 24 (solid line curve) of the right wheel indiagram B is correspondingly steep while the deceleration curve 25(dotted line curve) of the left wheel is flat and does not reach thedeceleration level value -b,. According to the invention the controlvalve is closed at point I as represented in the curve 26 of diagram Cby a signal produced by the control system when a rotative speed has achange of at least Av and a rotative speed difference A between the leftand the right wheel which is determined after the right wheel hasexceeded the deceleration level b,. The

right wheel can now reaccelerate following the pressure decrease in thepart of the brake circuit connected with the wheel cylinders. As theleft wheel was hardly decelerated there is no distinct reaccelerationmaximum in its deceleration curve 25. The electronic control system ofthis invention produces a signal for the opening of the control valve(point II in curve 26) no speed decrease Av having been registeredbefore at the left wheel as soon as the wheel which previously showedgreat deceleration, in our example the right one, reaches itsreacceleration maximum b Then a new braking and a new control cycle canbegin. Referring to FIG. 4, there is illustrated therein in schematicblock diagram form one embodiment of the electronic control system 60incorporated in an antiskid system including therein a braking pressurecontrol valve 61 actuated by first and second actuation signals producedby control system 60. It is tobe understood that control system 60 andvalve 61, which may be a solenoid actuated 3/2 way valve is onlypresented as an example of one form of an anti-skid system that enablesachievement of the objects and features of the present invention. Othertypes of electronic control systems and control valves may'be providedto achieve the object and features of the present invention.

Prior to the actuation of control system 60, control valve 61 passeshydraulic fluid from driver actuated braking device 62 which may be inthe form of a known master cylinder or a combination of a mastercylinder and power brake (brake booster) actuated by a foot pedal. Thehydraulic fluid passes from valve 61 to the right wheel brake 63 of anaxle of a motor vehicle and to the left wheel brake 64 of the same axle.

The electronic control system will now be described. Speed sensor 31 isassociated with the right wheel of one of the vehicle axles to determinethe rotary speed thereof and may take any number of different formsknown in the art. A similar speed sensor 32 is associ ated with the leftwheel of the same axle of the vehicle. The output of sensors 31 and 32are coupled to speed decrease detectors 33 and 34, respectively.Detectors 33 and 34 may be a threshold type circuit that produces anoutput signal when the threshold bias is exceeded wherein the thresholdbias will determine the change of speed Av that is to be detected.Incipient skid deceleration detectors 35 and 36 are coupled to speedsensors 31 and 32, respectively. Detectors 35 and 36 may include, forinstance, an integrator coupled to the speed sensor, a differentiatorcoupled to the output of the integrator and a threshold device coupledto the output of the differentiator. The threshold level of thethreshold device of detectors 35 and 36 would be set to correspond tothe bs deceleration level illustrated in diagrams B of FIGS. 1-3 so thatan output indicating an incipient skid is produced when the decelerationsignal exceeds the threshold level of the threshold device.Reacceleration detectors 37 and 38 are coupled to sensors 31 and 32,respectively. Detectors 37 and 38 may include, for instance, anintegrator coupled to the sensor, a differentiator coupled to the outputof the integrator and a threshold device coupled to the output of thedifferentiator with the threshold level of the threshold device beingset at a value equal to b for the particular wheel being considered.Detectors 37 and 38 will produce an output when the threshold level settherein is equaled or exceeded. Further a subtractor 39 is coupled tothe output of each of the sensors 31 and 32. Subtractor 39 provides anoutput signal when there is a significant difference in the rotary speedof the two wheels as sensed by sensors 31 and 32.

In order to obtain a first actuation signal for pressure release in thebrake circuit in the normal case illustrated in FIG. 1, that is, whenboth wheels approach the incipient skid level -bs approximately at thesame time, output signals of detectors 33 and 35 are coupled to AND gate41 and the output signals from detectors 34 and 36 are coupled to ANDgate 42. When there is input signal from the two detectors coupled toAND gates 41 and 42 there will be provided an output signal coupled tostorage or time delay devices 43 and 44, respectively. The output signalfrom devices 43 and 44 are coupled to an AND gate 40 which provides thefirst actuation signal for valve 61 through OR gate 45. Storage or timedelay devices 43 and 44 may take many different forms so as to store ortime delay the presence of an output signal from their associated one ofgates 41 and 42. One such form could include a bistable device for eachof the devices 43 and 44 which is set to provide a high or binary 1output when an input signal is received from its associated one of gates41 and 42.

Prior to starting another control cycle the bistable devices of devices43 and 44 would be reset to its low or binary 0 output by the secondactuation signal whose production will be described hereinbelow.

In accordance with the condition illustrated in FIG. 2, wherein highfriction value differences are present at the two wheels, the speeddecrease of one of the wheels is very small, or according to FIG. 3 thedeceleration valve bs has not been reached, one of the AND gates 41 or42 does not emit a signal. In order to provide a first actuation signalto prevent skidding of the wheel that is in a condition to skid two ANDgates 46 and 47 are provided. One input to AND gate 46 is coupled totime delay device 44 and the other input thereof is connected to theoutput of subtractor 39. One input of AND gate 47 is coupled to theoutput of time delay device 43 and the other input thereof is connectedto the output of subtractor 39. As soon as a wheel reaches thedeceleration level bs and a certain speed decrease is detected at one ofthe wheels, in other words, one of the wheels tends to skid, and thereis a certain speed difference beteen the two wheels one of the AND gates46 and 47 produces an output signal which again passes through OR gate45 as the first actuation signal to actuated valve 61 for pressurerelease in the brake circuit. As described hereinabove with respect toFIGS. 1 and 2, the second actuation signal to enable a new pressurebuildup in the wheel cylinders, where both wheels have previouslyexceeded the deceleration level --bs during the braking is produced whenboth wheels have reached their associated reacceleration maximum levelafter the pressure release. For the purpose of generating the secondactuating signal under these conditions the output signals of time delaydevices 43 and 44 are coupled to an OR gate 48. The output signal of ORgate 48 togetherwith the output signals of detectoRs 37 and 38 arecoupled to an AND gate 49. An output signal from AND gate 49 when bothdetectors 37 and 38 produce an output signal and there is an outputsignal from either of devices 43 and 44 becomes the second actuationsignal for valve 61 at the output of OR gate 50 which enables a newpressure buildup in the brake system connected to the two wheels.Simultaneously this second actuation signal will reset time delaydevices 43 and 44 to enable the start of the next control cycle.

In the situation illustrated in FIG. 3 one of the wheels does not reachthe deceleration level -bs during braking, but the first actuationsignal was produced only because of the other wheel tended to skid. Anew pressure buildup which is permitted by the generation of the secondactuating signal is produced as a function of the reacceleration maximumof the wheel that has reached or exceeded the deceleration level -bsduring the braking process and then has reached its reaccelerationmaximum value. To accomplish the production of the second actuationsignal in the situation illustrated in FIG. 3 the output signals of thetwo time delay devices 43 and 44 are coupled to NOT gates 51 and 52,respectively. The output signal of NOt gate 51, which is associated withthe right wheel, is coupled to AND gate 53, which is associated with theleft wheel. The other input signals to AND gate 53 are provided by theoutput signal of time delay device 44 and detector 38. When these threeinput signals to AND gate 53 are high an output signal is produced whichbecomes the second actuation signal for valve 61 at the utput of OR gate50. Simultaneously, this second actuation signal resets devices 43 and44. The output signal of NOT gate 52, which is associated with the leftwheel, is coupled to AND gate 54, which is associated with the rightwheel, the other input signals for AND gate 54 are the output signals ofthe delay device 43 and detector 37. When these three input signals toAND gate 54 arehigh an output signal is produced which becomes thesecond actuation signal for valve 61 at the output of OR gate 50.Simultaneously, this second actuation signal resets devices 43 and 44.The second actuation signal is produced by the output signal of ANDgates 53 or 54 depending upon which of the wheels exceeds thedeceleration level -bs'and then achieves its maximum reaccelerationlevel.

ln accordance with the principles of the present invention the responseof the electronic control circuit is in accordance with an average ofthe control values of both wheels, when only one braking pressurecontrol valve is arranged in the common brake circuit, so that ashortened braking distance is achieved without the danger that theincipient skid deceleration level is exceeded and undesired skiddingwith take place.

While we have described above the principles of our invention inconnection with specific apparatus it is to be more clearly understoodthat this description is made only by way of example and not as alimitation to the scope of our invention as set forth in the objectsthereof and in the accompanying claims.

We claim:

1. An anti-skid system for a motor vehicle comprising:

a single brake pressure control means to control the braking of bothwheels of at least one axle of said vehicle; and 1 an electronic controlsystem to produce at least a firs actuation signal for said controlmeans to release brake pressure at both of said wheels to prevent theskidding of at least one of said wheels;

said control system including a pair of speedsensors, a different one ofsaid sensors being coupled to each wheel of said one axle,

a subtractor coupled to each of said sensors to produce a first signalproportional to the difference in speed signals generated by both ofsaid sensors,

a first incipient'skid deceleration detector coupled to one of saidsensors to produce a second signal proportional to an incipientskiddeceleration signal of one of said wheels,

a second incipient skid deceleration detector coupled to the other ofsaid sensors to produce a third signal proportional to an incipient skiddeceleration signal of the other of said wheels,

a first speed decrease detector coupled to one of said sensors toproduce a fourth signal proportional to a given decrease in one of saidspeed signals after said second signal is produced,

a second speed decrease detector coupled to the other of said sensors toproduce a fifth signal proportional to a given decrease in the other ofsaid speed signals after said third signal is produced, and

first logic circuitry coupled to said subtractor, said first and seconddeceleration detectors and said first and second decrease detectors toproduce said first actuation signal in response to a combination ofsignals selected from said first, second, third, fourth and fifthsignals.

2. A system according to claim 1, wherein said first logic circuitry isresponsive to the combination of said second, third, fourth and fifthsignals to produce said first actuation signal.

3. A system according to claim 1, wherein said first logic circuitry isresponsive to the combination of said first signal, one of said secondand third signals and one of said third and fourth signals to producesaid first actuation signal.

4. A system according to claim 1, wherein said first logiccircuitry'includes a first AND gate coupled to said first decelerationdetector and said first speed decrease detector,

a second AND gate coupled to said second deceleration detector and saidsecond speed decrease detector,

a first time delay device coupled to said first AND gate,

a second time delay device coupled to said second AND gate,

a third AND gate coupled to said first and second time delay devices,

a fourth AND gate coupled to said second time delay device and saidsubtractor,

a fifth AND gate coupled to said first time delay device and saidsubtractor, and

a first OR gate coupled to said third, fourth and fifth AND gates toproduce said first actuation signal.

5. A system according to claim 4, wherein said control system furtherincludes means coupled to both of said sensors and said control means toproduce a second actuation signalto cause said control means to reapplybraking pressure to both of said wheels.

6. A system according to claim 5, wherein said second actuation signalis produced when both of said wheels have achieved a givenreacceleration value.

7. A system according to claim 5, wherein said second actuation signalis produced when the speed of one of said wheels does not result inproducing one of said second and third signals and the other of saidwheels has achieved a given reacceleration value after producing theother of said second and third signals. 1

8. A system according to claim 5, wherein said means includes a firstreacceleration detector coupled to one of said sensors to produce asixth signal when the associated one of said wheels has achieved a firstgiven reacceleration value,

a second reacceleration detector coupled to the other of said sensors toproduce a seventh signal when the associated one of said wheels hasachieved a second given-reacceleration value, and second logic circuitrycoupled to said first and second reacceleration detectors and said firstand second time delay devices to produce said second actuation signal.

9. A system according to claim 8, wherein said logic circuitry includesa second OR gate coupled to said first and second time delay devices, Ia first NOT gate coupld to said first time delay device, a second NOTgate coupled to said second time delay device,

a sixth AND gate coupled to said second time delay device, said firstNOT gate and said second reacceleration detector,

a seventh AND gate coupled to said first time delay device, said secondNOT gate and said first reacceleration detector,

an eighth AND gate coupled to said second OR gate and said first andsecond reacceleration detectors, and

a third OR gate coupled to said sixth, seventh and eighth AND gates toproduce said second actuation signal.

10. A system according to claim 9, wherein the output of said third ORgate is coupled to said first and second time delay device, said firstand second time delay device being reset to their initial outputcondition in response to said second actuation signal.

11. A system according to claim 1, wherein said control system furtherincludes means coupled to both of said sensors and said control means toproduce a second actuation signal for said control means as a functionof reacceleration signals generated in response to a speed signal fromboth of said sensors, said second actuation signal causing said controlmeans to reapply braking pressure to both of said wheels.

12. A system according to claim 11, wherein said second actuation signalis produced when both of said wheels have achieved a givenreacceleration value.

13. A system according to claim 11, wherein said second actuation signalis produced when one of said wheels has not achieved an incipient skiddeceleration and the other of said wheels has achieved a givenreacceleration value after having achieved an incipient skiddeceleration.

1. An anti-skid system for a motor vehicle comprising: a single brakepressure control means to control the braking of both wheels of at leastone axle of said vehicle; and an electronic control system to produce atleast a first actuation signal for said control means to release brakepressure at both of said wheels to prevent the skidding of at least oneof said wheels; said control system including a pair of speed sensors, adifferent one of said sensors being coupled to each wheel of said oneaxle, a subtractor coupled to each of said sensors to produce a firstsignal proportional to the difference in speed signals generated by bothof said sensors, a first incipient skid deceleration detector coupled toone of said sensors to produce a second signal proportional to anincipient skid deceleration signal of one of said wheels, a secondincipient skid deceleration detector coupled to the other of saidsensors to produce a third signal proportional to an incipient skiddeceleration signal of the other of said wheels, a first speed decreasedetector coupled to one of said sensors to produce a fourth signalproportional to a given decrease in one of said speed signals after saidsecond signal is produced, a second speed decrease detector coupled tothe other of said sensors to produce a fifth signal proportional to agiven decrease in the other of said speed signals after said thirdsignal is produced, and first logic circuitry coupled to saidsubtractor, said first and second deceleration detectors and said firstand second decrease detectors to produce said first actuation signal inresponse to a combination of signals selected from said first, second,third, fourth and fifth signals.
 2. A system according to claim 1,wherein said first logic circuitry is responsive to the combination ofsaid second, third, fourth and fifth signals to produce said firstactuation signal.
 3. A system according to claim 1, wherein said firstlogic circuitry is responsive to the combination of said first signal,one of said second and third signals and one of said third and fourthsignals to produce said first actuation signal.
 4. A system according toclaim 1, wherein said first logic circuitry includes a first AND gatecoupled to said first deceleration detector and said first speeddecrease detector, a second AND gate coupled to said second decelerationdetector and said second speed decrease detector, a first time delaydevice coupled to said first AND gate, a second time delay devicecoupled to said second AND gate, a third AND gate coupled to said firstand second time delay devices, a fourth AND gate coupled to said secondtime delay device and said subtractor, a fifth AND gate coupled to saidfirst time delay device and said subtractor, and a first OR gate coupledto said third, fourth and fifth AND gates to produce said firstactuation sIgnal.
 5. A system according to claim 4, wherein said controlsystem further includes means coupled to both of said sensors and saidcontrol means to produce a second actuation signal to cause said controlmeans to reapply braking pressure to both of said wheels.
 6. A systemaccording to claim 5, wherein said second actuation signal is producedwhen both of said wheels have achieved a given reacceleration value. 7.A system according to claim 5, wherein said second actuation signal isproduced when the speed of one of said wheels does not result inproducing one of said second and third signals and the other of saidwheels has achieved a given reacceleration value after producing theother of said second and third signals.
 8. A system according to claim5, wherein said means includes a first reacceleration detector coupledto one of said sensors to produce a sixth signal when the associated oneof said wheels has achieved a first given reacceleration value, a secondreacceleration detector coupled to the other of said sensors to producea seventh signal when the associated one of said wheels has achieved asecond given reacceleration value, and second logic circuitry coupled tosaid first and second reacceleration detectors and said first and secondtime delay devices to produce said second actuation signal.
 9. A systemaccording to claim 8, wherein said logic circuitry includes a second ORgate coupled to said first and second time delay devices, a first NOTgate coupld to said first time delay device, a second NOT gate coupledto said second time delay device, a sixth AND gate coupled to saidsecond time delay device, said first NOT gate and said secondreacceleration detector, a seventh AND gate coupled to said first timedelay device, said second NOT gate and said first reaccelerationdetector, an eighth AND gate coupled to said second OR gate and saidfirst and second reacceleration detectors, and a third OR gate coupledto said sixth, seventh and eighth AND gates to produce said secondactuation signal.
 10. A system according to claim 9, wherein the outputof said third OR gate is coupled to said first and second time delaydevice, said first and second time delay device being reset to theirinitial output condition in response to said second actuation signal.11. A system according to claim 1, wherein said control system furtherincludes means coupled to both of said sensors and said control means toproduce a second actuation signal for said control means as a functionof reacceleration signals generated in response to a speed signal fromboth of said sensors, said second actuation signal causing said controlmeans to reapply braking pressure to both of said wheels.
 12. A systemaccording to claim 11, wherein said second actuation signal is producedwhen both of said wheels have achieved a given reacceleration value. 13.A system according to claim 11, wherein said second actuation signal isproduced when one of said wheels has not achieved an incipient skiddeceleration and the other of said wheels has achieved a givenreacceleration value after having achieved an incipient skiddeceleration.